Buggy code:
public class JavassistBasedClassBuilder implements ClassBuilder { @ Override public Class < ? > buildClass ( DTODescription description , String proxyClassName ) throws CannotBuildClassException { ClassPool classPool = ClassPool . getDefault ( ) ; Class < ? > entityClass = description . getType ( ) ; CtClass hyalineProxyClass = classPool . makeClass ( proxyClassName ) ; if ( Modifier . isFinal ( entityClass . getModifiers ( ) ) ) { throw new CannotBuildClassException ( ( ( "Cannot<seq2seq4repair_space>proxy<seq2seq4repair_space>class<seq2seq4repair_space>" + ( entityClass . getCanonicalName ( ) ) ) + "<seq2seq4repair_space>since<seq2seq4repair_space>it<seq2seq4repair_space>is<seq2seq4repair_space>final." ) ) ; } CtClass clazz = null ; try { clazz = classPool . get ( entityClass . getName ( ) ) ; } catch ( NotFoundException e ) { classPool . appendClassPath ( new ClassClassPath ( entityClass ) ) ; try { clazz = classPool . get ( entityClass . getName ( ) ) ; } catch ( NotFoundException e1 ) { throw new CannotBuildClassException ( e1 . getMessage ( ) ) ; } } try { hyalineProxyClass . setSuperclass ( clazz ) ; } catch ( CannotCompileException e1 ) { throw new CannotBuildClassException ( e1 . getMessage ( ) ) ; } for ( Class < ? > interfaceType : description . getImplementedInterfaces ( ) ) { CtClass anInterface = null ; try { anInterface = classPool . get ( interfaceType . getName ( ) ) ; } catch ( NotFoundException e ) { classPool . appendClassPath ( new ClassClassPath ( interfaceType ) ) ; try { anInterface = classPool . get ( interfaceType . getName ( ) ) ; } catch ( NotFoundException e2 ) { throw new CannotBuildClassException ( e2 . getMessage ( ) ) ; } } hyalineProxyClass . addInterface ( anInterface ) ; } ClassFile ccFile = hyalineProxyClass . getClassFile ( ) ; ConstPool constpool = ccFile . getConstPool ( ) ; addClassAnnotations ( description , hyalineProxyClass , constpool ) ; CtField targetField ; try { <START_BUG> targetField = CtField . make ( ( ( "private<seq2seq4repair_space>" + ( description . getTarget ( ) . getClass ( ) . getName ( ) ) ) + "<seq2seq4repair_space>target;" ) , hyalineProxyClass ) ; <END_BUG> hyalineProxyClass . addField ( targetField ) ; for ( FieldDescription field : description . getFields ( ) . values ( ) ) { int modifiers = field . getField ( ) . getModifiers ( ) ; if ( ( Modifier . isPrivate ( modifiers ) ) || ( ( ! ( Modifier . isPrivate ( modifiers ) ) ) && ( ! ( Modifier . isFinal ( modifiers ) ) ) ) ) { CtField ctField = createFieldFromDescription ( classPool , constpool , field , hyalineProxyClass ) ; hyalineProxyClass . addField ( ctField ) ; if ( field . isFromTemplate ( ) ) { CtMethod getter = createGetter ( field , hyalineProxyClass ) ; hyalineProxyClass . addMethod ( getter ) ; CtMethod setter = createSetter ( field , hyalineProxyClass ) ; hyalineProxyClass . addMethod ( setter ) ; } } } for ( MethodDescription method : description . getMethods ( ) . values ( ) ) { String methodName = method . getMethod ( ) . getName ( ) ; if ( ! ( Modifier . isFinal ( method . getMethod ( ) . getModifiers ( ) ) ) ) { if ( ( ( methodName . startsWith ( "get" ) ) || ( methodName . startsWith ( "set" ) ) ) || ( methodName . startsWith ( "is" ) ) ) { CtMethod ctMethod ; try { ctMethod = createMethodFromDescription ( classPool , constpool , method , hyalineProxyClass , description ) ; if ( ctMethod != null ) { hyalineProxyClass . addMethod ( ctMethod ) ; } } catch ( FieldNotFoundException e ) { e . printStackTrace ( ) ; } } } } return hyalineProxyClass . toClass ( ) ; } catch ( CannotCompileException e ) { throw new CannotBuildClassException ( e . getMessage ( ) ) ; } } private CtMethod createMethodFromDescription ( ClassPool classPool , ConstPool constpool , MethodDescription method , CtClass hyalineProxyClass , DTODescription description ) throws CannotCompileException , CannotBuildClassException , FieldNotFoundException { } private void addClassAnnotations ( DTODescription description , CtClass hyalineProxyClass , ConstPool constpool ) throws CannotBuildClassException { } private CtField createFieldFromDescription ( ClassPool classPool , ConstPool constpool , FieldDescription field , CtClass hyalineProxyClass ) throws CannotCompileException , CannotBuildClassException { } private CtMethod createSetter ( FieldDescription field , CtClass hyalineProxyClass ) throws CannotCompileException , CannotBuildClassException { } private CtMethod createGetter ( FieldDescription field , CtClass hyalineProxyClass ) throws CannotCompileException , CannotBuildClassException { } }

Fix:
targetField = CtField . make ( ( ( "private<seq2seq4repair_space>" + ( description . getType ( ) . getName ( ) ) ) + "<seq2seq4repair_space>target;" ) , hyalineProxyClass ) ;